Nowadays, several technologies exist to bring analogue services and digital internet protocol (IP) services to a user in, e.g., a home environment, enterprise environment, public environment, etc. trough broadcasting, multicasting, and/or unicasting over wireless networks, terrestrial networks, satellite networks, coaxial cable networks, etc. IPTV is an example of an IP service. It is a video service offered by a service provider that owns a network infrastructure and controls the content distribution to its customers. Typically, IPTV services run over a multi-service IP network/infrastructure which can provide data, voice and video on the same network/infrastructure. Usually, the information included in the services is protected against unauthorized access/usage. As an example, broadcast/multicast protection systems normally operate with a number of distinguished steps. A service registration step is usually required in which a user enters an agreement with a service provider in a communication network i.e. the user becomes a subscriber or customer. In this step, the user is e.g. provided with a personal, unique and secret subscription key which may be provided as a hard or soft token, e.g. in the form of a smart-card, or software certificate. In a key-distribution step, one or several information protection key(s) is/are distributed (e.g. protected by the subscription key) to registered users for decryption of the protected information. The service provider in the network encrypts the protected information using these information protection keys in a service delivery protection step. In addition, the service provider can perform a re-keying in order to update the protected information key(s), e.g. when a new user is registered or when a user de-registers or when a key is compromised. The service provider may also perform a periodic re-keying to further increase the security of the system. In for example a home environment, a family/household is generally considered to have a common subscription for all users in the home. This is the case if the service(s) is an IPTV channel or a TV channel is general. But even if a certain service is included in a family's subscription, this does not mean that any device/user should be able to access to the service. For instance, it is desirable to protect children from certain types of content. Some other (high-value) content may be allowed to be played on certain trusted (e.g. high security level) devices, or only on a limited number of devices, even for other services. Since only a limited number of devices (or group of devices) may be allowed to have access to certain services e.g. in a home environment, all the services provided to the group in the home environment should not be encrypted with the same key. Otherwise, all devices of the group owning the key will be able to have access to all services in the domain.
There are well known DRM (Digital Rights Management) techniques which can be used to implement access control in such groups by encrypting content and controlling which users/devices have access to the decryption key. When a user/device either leaves or joins a service group it is necessary to change the key; a process associated with overhead. For example, if a single user/device leaves the service group, all remaining users/devices must typically be re-keyed, and each such re-key operation requires signalling, cryptographic operations etc.
In for example a typical IPTV scenario, a common event is that a user stops watching one channel and switches to another channel, i.e. the user moves between an old and a new service group. A drawback with having to update the key(s) every time a status of a user changes i.e. every time a user/device leaves or joins a group, hereinafter denoted a service group, where services are available for the user, is that both the key(s) of the old service (or old group of services) and the key(s) of the new service (or new group of services) has/have to be updated. Such re-keying or updating of the keys increases the computational load, affects negatively the quality of service, etc., not only for the device of the user that leaves or joins the group but also for all other devices that have access to the old service and the new service(s) or group of services. In other words, as discussed above, the number of users/devices to be given a new key is proportional to the size of the whole service group, even if only a single user leaves/joins the service group. The user that leaves or joins a service group will force non-leaving or non-joining users to undergo the key management process. The problem is even more pronounced when we have a public environment where a service group can have hundreds or even thousands of users/devices, since forcing all users of the group to undergo the key management will cause large overhead and computation load and will affect the quality of service perceived by the users (e.g. unnecessary lags, interruptions etc.).
There exist key management schemes/approaches proposed that can reduce the computational load such as group key distribution protocols used to improve scalability. The following prior art documents describe key management schemes:    LKH (Logical Key Hierarchy) defined in a RFC 2627 report entitled: “Key Management for Multicast and Architectures” issued by the standardization organisation Internet Engineering Task Force (IETF) issued June 1999.    SD (Subset Difference) described in a document by Naor, and Lotspiech and entitled “Revocation and Tracing scheme for stateless receivers”, manuscript 2001.    LSD (Layered Subset Differences). described in “The LSD Broadcast Encryption Scheme” by Dani Halevy, Adi Shamir, Proceedings of CRYPTO 2002: pp 47-60
However, the number of key management operations known from the prior art, still depends on the group size. For example, typically in a group of e.g. N users (N can take any value), the number of key management operations will grow like log (N). While this is a great improvement compared to a linear dependency of N, it will still depend on the group size. Also, these described schemes require their own administration overhead which may not pay off at all unless N is very large. In particular, if a user sequentially moves between different service groups, the overhead will even be dependent on the total number of users accumulated over all m groups (m can take any value larger than 1). Furthermore, for the e.g. log(N) affected devices, the disturbances will still occur every time a device joins or leaves their group. In other words, the above-mentioned schemes succeed in reducing the number of affected devices, not in reducing the gravity of the disturbance.
Another known approach to reduce the computation load is to pre-distribute keys to users/devices. However, this would mean that devices/users are in possession of keys that are perhaps unnecessary, leading to unwanted increased risk of key exposure thus affecting the secrecy of the individual services/groups of services. Moreover, the number of required keys is considered in general to be exponential to the number of users to enable all possible configurations of users into groups making memory/storage space in user devices an issue. A known key management method is described in European patent application number EP 1549010A1. In this prior art document a inter-area rekeying of encryption keys in secure mobile multicast communications is disclosed, in which a domain group controller key server distributes traffic encryption keys to local group controller key servers serving respective group key management areas. According to this prior art, no local rekeying is triggered whenever a mobile current group member moves between two areas of the group. This is performed to avoid rekeying when a group member moves within the group. But whenever a new member (as opposed to a current group member entering the area by intra-group mobility) joins the group, rekeying operation is performed.